Titanium dioxide TiO2 (simply referred to as titanium oxide herein and in the claims) has hitherto been known as a substance showing a photocatalytic function. As methods for forming a titanium oxide film on a titanium metal, there have been known since the 1970s a method for forming a titanium oxide film on a titanium metal by anodic oxidation, a method for thermally forming a titanium oxide film on a titanium metal plate in an electric furnace supplied with oxygen, and a method for forming a titanium oxide film on a titanium metal by heating a titanium plate in flames of a city gas at 1,100 to 1,400° C. (see non-patent document 1). Numerous studies designed to achieve the practical use of photocatalysts have been conducted in many technical fields.
To manufacture photocatalyst products for obtaining a deodorizing, antimicrobial, anti-fogging, or antifouling effect by such a photocatalytic function, it has been common practice to impart a titanium oxide sol onto a substrate by spray coating, spin coating, or dipping, thereby forming a film. However, the resulting film is apt to peel off or wear, and thus its long-term use has been difficult.
Ultraviolet radiation with a wavelength of 400 nm or less is necessary for titanium oxide to function as aphotocatalyst, but many studies have been performed on titanium oxide photocatalysts which are doped with various elements to function by visible light. For example, there is a report comparing titanium oxides doped with, for example, F, N, C, S, P and Ni, and showing the nitrogen-doped titanium oxide to be excellent as a visible light responding photocatalyst (see non-patent document 2).
As titanium oxide photocatalysts doped with other elements as shown above, proposals were made for a photocatalyst comprising a titanium compound Ti—O—X having the oxygen site of titanium oxide substituted by an atom X such as nitrogen, or an anion X, a photocatalyst comprising a titanium compound Ti—O—X having an atom X such as nitrogen, or an anion X, doped in the spaces of the crystal lattice of titanium oxide, and a photocatalyst comprising a titanium compound Ti—O—X having an atom X such as nitrogen, or an anion X, disposed at the grain boundaries of polycrystalline aggregates of titanium oxide crystals (see patent documents 1 to 4).
A further report says that natural gas combustion flames with the temperature of combustion flames maintained in the vicinity of 850° C., for example, by adjusting the flow rates of a natural gas and/or oxygen were struck against a titanium metal to obtain chemically modified titanium oxide n-TiO2—xCx, which absorbed light at 535 nm or less (see non-patent document 3).
Patent document 1: Japanese Patent Application Laid-Open No. 2001-205103 (claims)
Patent document 2: Japanese Patent Application Laid-Open No. 2001-205094 (claims)
Patent document 3: Japanese Patent Application Laid-Open No. 2002-95976 (claims)
Patent document 4: International Publication 01/10553 brochure (claims)
Non-patent document 1: A. Fujishima et al., J. Electrochem. Soc. Vol. 122, No. 11, p. 1487-1489, November 1975
Non-patent document 2: R. Asahi et al., SCIENCE Vol. 293, Jul. 13, 2001, p. 269-271
Non-patent document 3: Shahed U. M. Khan et al., SCIENCE Vol. 297, Sep. 27, 2002, p. 2243-2245